Ionic Liquids as a Green Solvent


An important application of green chemistry can be seen in the development of green solvents, specifically ionic liquidsas green solvents. Ionic liquids are salts in liquid form at room temperature and are composed entirely of ionic species. They are excellent solvents for many reactions because they are ionic compounds and thus have negligible vapour pressure. Because of this they emit negligible dangerous by-products and can thus be considered "green". The green principles that these solvents represent include the reduction of toxic reagents, prevention of waste production and the use of energy-efficient processes. This concept is used in a variety of industries including pharmaceutical manufacturing, development of lubricants, paints, and cleaning materials as well as petrochemical applications. Petrochemical applications of green solvents include the extraction of petrochemicals (or in other words hydrocarbons) from crude oil, presently an important process in the oil and gas industry.

The chemical research of these products has tremendously increased in the last decade due to new government regulations on waste production and greenhouse gas emissions. Non-green solvents add to the damage of the earth’s atmosphere because they eventually oxidize and create carbon dioxide, a greenhouse gas with great potential impact on global warming. However due to this new research in recent years, ionic liquids have emerged as possible green solvents or in other words, environmentally benign solvents.

Below is a video from the University of Leicester, depicting the nature of ionic liquids, how they are produced and their applications/uses in the world today. - http://www.youtube.com/watch?v=UHMIWk2iIdI

















1 Conventional Solvents


Green chemistry is the development of chemical research and new engineering designs that promote the use of less hazardous materials and processes. This is done to minimize waste and the generation of detrimental materials during numerous chemical processes. This concept is becoming a very important concern in our society today due to an alarming increase in the deterioration of our environment.

Greenhouse Gas Emissions
Greenhouse Gas Emissions

1.1 What is a Conventional Solvent?

Conventional solvents, most of which are volatile organic compounds (VOCs) do not promote green chemistry principles and can in fact, greatly hinder the environment and our atmosphere. Many VOCs are dangerous to human health if often inhaled as well as to the environment. They have high vapor pressures at room temperature which results in a low boiling point. This causes large amounts of molecules to evaporate from the compound and enter into the surrounding air. This can contribute to the greenhouse gas effect and cause pollution in the atmosphere. Common solvents are found in dry cleaning products, paint thinners, nail polish removers, glue solvents, detergents, perfumes, and in chemical synthesis (extraction of crude oil).

1.2 Health Risks

greenhouse_gas.jpg
Greenhouse Gas Effect
There are many health risks associated with using conventional solvents and having them evaporate into the atmosphere. Severe risks include respiratory, allergic, or immune effects in humans. Some VOCs (styrene, limonene) react with nitrogen oxides or with ozone to produce new products which can cause sensory irritation symptoms.[1] Many VOCs contribute to the creation of smog and emission of greenhouse gases, thus increase the effects of global warming. Smog can also cause negative health effects in large doses such as eye, nose, throat irritation, damage to liver, kidney and central nervous system. Many organic compounds can cause cancer in animals and some are suspected in even causing cancer in humans. [2] Of course, the extent and nature of the health effect depends on many factors including length of time exposed or how close to the exposure one may be.

2 Ionic Liquids as a Solvent


Instead of conventional solvents, ionic liquids have emerged in the industry as a new, "greener" solvent.

2.1 Properties

Ionic liquids are organic salts in liquid form at room temperature and are composed entirely of ionic species. [3] They are excellent solvents for many organic reactions (because they, themselves are also organic) and the organic products can be removed from the ionic liquid by extraction. Because they are ionic compounds, they have negligible vapour pressure meaning [4] they emit little to no dangerous products such as volatile organic compounds (VOC) and can thus be considered "green". This provides other favorable characteristics with regards to green chemistry such as low volatility, generally low-toxicity, no flammability and high thermal stability. They are made up of at least two components, a positive charge (cation) and a negative charge (anion) that can be varied depending on the type of reaction that is occurring. One or both of these ions are large and thecation is non-symmetrical. These factors lower the melting point of the ionic liquid and cause them to melt without decomposing or vaporizing.
Ionic Liquid Model
Ionic Liquid Model


Varying these components implies designing these solvents with specific properties to carry out a certain reaction with an end product in mind. These specific properties include melting point, viscosity, density, and hydrophobicity, which can all be varied by simple changes to the structure of the ions. Thus, the term “designer solvents” has often been used to describe ionic liquids as their properties and behavior can be adjusted to optimize product yield, selectivity, solubility, and product separation.




2.2 Examples of Ionic Liquids

Ionic liquids come in two main categories, first is a simple salt which is made of a single anion and cation and second is a binary ionic liquid which are salts where an equilibrium is involved. For the binary systems, the melting points depend on composition (as stated above). Below are some examples of simple room temperature ionic liquids.

Picture_3.png
Simple Ionic Liquids


3 Reversal Mechanism


[5] The forward reaction is the conversion of two non-ionic liquids in mixture to be converted to an ionic liquid as CO2 gas is bubbled through them at moderate temperature and pressure. The newly formed ionic liquid can be reverted fully back to a non-ionic liquid by bubbling either N2 or Argon through the mixture at moderate temperature.

forward_reaction.png
Reaction of trialkylsilylamines with carbon dioxide to form reversible ionic liquids


The first reversible solvent to be discovered was a mixture between DBU (1,8-diazabicyclo-[5.4.0]-undec-7-ene) and 1-hexanol, although it is found that DBU and other alcohol mixtures exhibit the same reversibility. The selection of the alcohol is important as short chained alcohols will result in a solid after exposure to CO2, while longer chained alcohols result in ionic liquids.

3.1 Forward Reaction

The forward reaction is exothermic in nature. As CO2 is bubbled through DBU, the liquid quickly becomes viscous. This is due to the DBU being completely converted to the protonated DBUH, an ionic liquid.

3.2 Reverse Reaction

To reverse the reaction, N2 or Argon can be bubbled through the ionic liquid. Raising the temperature of the reaction to 50˚C, a fairly moderate temperature, can make a large change in the rate of reaction. In the reverse reaction, the viscosity drops greatly as the ionic liquid is fully converted back to 1-hexanol and DBU.

Untitled1.png
Figure 1

From Figure 1,
a - Protonation of DBU in the presence of an alcohol and C02 is reversed when C02 is removed.

b - Polarity switching in which C02 causes a nonpolar liquid (blue) mixture of hexanol and DBU to change into a polar, ionic liquid (red). N2 reverses the process by removing C02 from the reaction.

c - The different polarity of each liquid under the two conditions is shown by the miscibility of decane with the hexanol/DBU mixture under nitrogen, before exposure to C02. But, decane seperates out when the mixture becomes polar in the presence of C02. And again, N2 reverses the process.








3.3 Testing the Mechanism

The reactions were tested using an addition of n-Decane to a 1:1mol mixture of DBU and 1-hexanol in a vessel. n-Decane was miscible with the non ionic liquid at room temperature. [6] CO2 was then bubbled through the vessel resulting in a formation of two separate phases, the n-Decane and the resulting ionic liquid. Argon was then bubbled through the vessel, which resulted in the reformation of a one phase mixture. Changes of the liquids can be observed by 1H-NMR spectroscopy, where it is revealed that throughout the forward and reverse reactions, key protons undergo chemical shifts.

Although this is just one example of an ionic liquid’s mechanism, CO2 can be used as a trigger for many designer solvents. Such as, (3-aminopropyl)-triethoxysilane(TESA) which is used in the extraction of hydrocarbons from crude oil.

4 Benefits and Applications in Industry


4.1 Distillation

[7] Reversible ionic liquids can be utilized to reduce and often even eliminate the distillation process required in many chemical processes. Chemical processes involved in industry often have numerous reaction and separation steps. Typically, the solvent needed varies from one step to the next. Therefore, at the end of each step the solvent must be separated from the product stream via distillation in order for a new solvent to be added. This requires vast amounts of energy and time, and therefore is economically and environmentally disadvantageous. Distillation is often also required in order to salvage the solvent for repeat use. Some conventional solvents, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and hexamethylphosphoramide (HMPA), have extremely high boiling points and large amounts of energy are needed to carry out the distillation process. This expensive procedure can often be eliminated if the properties of the solvents can be adjusted at each step.
Distillation_apparatus.png
Laboratory Distillation Apparatus

A reversible ionic liquid can be changed from a molecular liquid to an ionic liquid in the presence of carbon dioxide and vice versa in the presence of nitrogen gas. The property change that is most beneficial when a liquid is changed from molecular to ionic is the change in polarity. When the polarity of the solvent is changed, the miscibility also changes, allowing solvents which are immiscible with the product stream to be easily decanted off. Reversible ionic solvents also reduce the number of times solvents need to be replaced in a specific chemical process because their molecular architecture can easily be modified. Solvents that can be adjusted to suit the needs of each step in the chemical process are often referred to as designer solvents.
Distillation_column.jpg
Typical Industrial Distillation Tower


4.2 Extraction of Hydrocarbons from Crude Oil

One prevailing application of reversible ionic liquids in the energy industry is the extraction of hydrocarbons from contaminated crude oil.

4.2.1 Conventional Approach

Bitumen.jpg
Crude Oil

Conventionally, oil-miscible organic solvents are used in the extraction of hydrocarbons from crude oil. However, the process is time-consuming and the solvent is unable to be recycled without using vast amounts of energy. In addition, the oil-miscible organic solvents are often very volatile and as such are unsafe to use in large quantities. Solvents in the form of reversible ionic liquids can be used to replace these organic solvents to reduce economic and environmental impact.


4.2.2 Alternative Approach using Ionic Liquids

Reversible ionic liquids can be used to extract and separate hydrocarbons without the need of distillation. [8] The extraction process begins with mixing a specific molecular liquid with the contaminated crude oil. Molecular liquids are typically amines, and chosen based on toxicity, pH level and commercial availability. For example, a suitable amine in this context is a trialkylsilylamine (TESA). Both the crude oil and the molecular liquid are non-polar substances and therefore form a single phase system. The purpose of the solvent is to reduce the viscosity of the crude oil making the filtration of contaminants much easier. Contaminants that are present in the crude oil include sand, inorganic salts and other inorganic species. Carbon dioxide is then bubbled through the solution and undergoes the aforementioned mechanism. Following the introduction of carbon dioxide gas to the solution, the molecular liquid changes to an ionic liquid which is polar in nature. This causes the separation of the crude oil and the now ionic liquid which is easily decanted off of the crude oil. In small scale experiment, crude oil at fifty percent weight, was added to the molecular liquid, TESA, and was successfully able to extract all of the hydrocarbons from the contaminated crude oil.
This experiment was performed three times using recycled TESA and the separation efficiency was deemed successful each time.The concept of using reversible ionic liquids to extract hydrocarbons from crude oil is especially important when dealing with heavy oils. Traditionally, large quantities of steam are used to reduce the viscosity of heavy oils, but as stated earlier, this is a extremely energy-intensive process which is economically very limited.

Bitumen_2.jpg
Bitumen

4.3 Scale-Up Simulation

Currently, this process has only been performed on small scale operations, however, in a scale-up simulation favourable results were obtained. Using a conceptual design program, tar sands containing eighty percent sand and the remainder hydrocarbons, by weight, were added to the molecular liquid, TESA. During filtration, all the of the sand was separated from the oil. Bubbling carbon dioxide through the hydrocarbon/solvent mixture showed that only four percent, in moles, of the solvent remained in the hydrocarbon product stream. The remainder of the solvent was able to be recovered and reused as the initial molecular liquid. This scale-up simulation illustrates that the molecular liquid was successful in extracting the hydrocarbons from the tar sand and also successful at keeping its integrity throughout the process.
The process described above also results in favourable economic advantages. In a scale up simulation, TESAC and the current steam separation methods were tested and compared. Tesac’s ability to separate oil from tar sands exceeded that of a steam separation refinery, producing 58 million barrels of crude oil per year, assuming 80% run time for the year. Steam separation was found to produce 40 million barrels per year. TESAC was determined to result in a much cheaper method of separating oil from tar sands compared to steam separation, as the cost oil manufactured were found to be roughly $0.70 per barrel, and $25.12 per barrel respectively. Most of the cost from using TESAC is from energy required in order for the reversal step to occur, as it requires a temperature of 110°C.


5 Evaluation of Ionic Liquids as a Green Solvent


DownloadedFile.jpegTo evaluate the ‘greenness’ of a solvent, one must first set forth a set of relevant criteria. Conventionally, the use of solvents has been evaluated solely on the environmental effects of the solvent itself. Clearly this is inadequate, for it does not take into account the process used to create the solvent or its effectiveness when applied to a specific process. For this reason, the solvents under consideration will also be evaluated by their use and synthesis. [9] Jessop classifies methods of solvent evaluation into two types: (1) A general comparison between solvents.(2) Specific comparisons within the scope of a specific application.
The former method considers the energy costs associated with the production of the solvent along with its environmental impact while the latter considers which solvent makes a specific process as a whole greener. As will be shown here, both factors must be taken into account in order to obtain a more accurate evaluation of a proposed green solvent.


5.1 General Comparison

At first glance, reversible ionic liquids may not seem like a plausible candidate for a green solvent. In terms of the general comparison method described above, ionic liquids are often more hazardous than conventional solvents. For instance, [10] Zhang et. al (2007) performed a life cycle assessment on ionic liquid 1-butyl-3-methyl-imidazolium tetrafluoroborate and compared the results to literature data for 4 conventional solvents. 11 factors were used, which included both the synthesis of the solvent and the effects of the solvent itself. The ionic liquid fared the worst in every category except ozone depletion. However, in order to obtain a broader understanding of the environmental impact of the solvent, the process for which the solvent is used must also be considered. It is entirely possible that a solvent which is not particularly green according to a general comparison may make a process as a whole more green. Such is the case with many reversible ionic liquids.
large_image.jpg

5.2 Comparisons within the scope of an application

The main advantage of reversible ionic liquids is the amount of energy that they save. The most common method of separating solvents from products is through distillation. This process takes advantage of the difference in vapor pressures of the components, which provides a means of separation. Unfortunately this method has many problems, the first of which is the large amounts of energy required. The minimum amount of energy required for distillation can be calculated from equation (1), where m is the mass of the sample, Cp is the average heat capacity, and ∆Hvap is the heat of vaporization. The second problem with distillation is that the solvent is required to be highly volatile. This is problematic, since volatile organic solvents are often toxic and flammable. Ionic liquids, on the other hand, have extremely low volatility, high thermal stability, and can be “designed with a particular end use in mind, or to possess a particular set of properties” (Earle & Seddon, 2000). Furthermore, by choosing the correct ionic liquid properties, high quantities of product yields can be produced as well as a reduced amount of undesired waste. Ionic liquids can often be recycled as well and thus leads to a reduction in process costs. Furthermore, the reactions are not difficult to carry out when using these solvents and no specific apparatus is required.
Q = mCp(Tb – Ti) + mΔHvap (1)

In the specific case of distillation being used to separate solvents from hydrocarbon fractions, another difficulty is encountered since the low-end carbon fractions are often distilled along with the solvent. The loss of these valuable hydrocarbons is a significant cost. For the reasons described above, it would be extremely desirable to eliminate distillation from the process with an alternative separation method. With the CO2 triggering mechanism described above, reversible ionic liquids provide a means of solvent extraction without distillation, greatly reducing the cost of the process. Overall, the use of ionic liquids leads to a greener process and as shown, is an immense improvement over current technology.


References


  1. ^ Bernstein, J. A.; Alexis, N.; Bacchus, H.; Bernstein, I. L.; Fritz, P.; Horner, E.; Li, N; Mason, S et al. (2008). "The health effects of nonindustrial indoor air pollution". Journal of Allergy and Clinical Immunology, 2008, 585–591.
  2. ^ United States Environmental Protection Agency. Volatile Organic Compounds.
    http://en.wikipedia.org/wiki/File:Greenhouse_Gas_by_Sector.png (accessed Nov. 14, 2011).
  3. ^ Earle, M.J.; Seddon, K.R. Ionic Liquids: Green Solvents for the Future. Pure Appl. Chem., Vol 72, No. 7, 2000, 1391-1398.
  4. ^ Zhao, H.; Xia, S.; Ma, P. Use of Ionic Liquids as "Green" Solvents for Extractions. Journal of Chemical Technology and Biotechnology., Vol 80, 2005, 1089-1096.
  5. ^
    Phan, L.M. C02-Triggered Switchable Solvent Systems and Their Applications.
    http://qspace.library.queensu.ca/bitstream/1974/1563/1/Phan_Lam_N_200802_Msc.pdf (accessed Nov. 10, 2011).
  6. ^
    Jessop, P. G.; Heldebrant, D. J.; Xiaowang, L.; Eckert, C. A.; Liotta, C. L. Reversible Nonpolar-to-polar Solvent. Nature [online] 2005, 436, 1102. http://www.nature.com.ezproxy.lib.ucalgary.ca/nature/journal/v436/n7054/full/4361102a.html (accessed Oct. 1. 2011)
  7. ^ Eckert, C. and Liotta, C. Eckert-Liotta Joint Research Group: Smart Solvents for Benign Separations. http://eckert.chbe.gatech.edu/pdf/Smart%20Solvents.pdf (accessed Nov. 1, 2011).
  8. ^ Blasucci, V.; Hart R.; Mestre V. L.; Hahne, D. J.; Burlager, M.; Huttenhower, H.; Thio, B. J. R.; Pollet, P.; Liotta, C. L.; Eckert, C. A. Single Component, Reversible Ionic Liquids for Energy Applications. Fuel 89. 2010, 1315-1319
  9. ^ Jessop, P. G. Searching for Green Solvents. Green Chemistry, 2011, 1391 - 1398.
  10. ^ Zhang, Y., Bakshi, B. R., & Demessie, E. S. Life Cycle Assessment of an Ionic Liquid versus Molecular Solvents and Their Applications. Environmental Science and Technology, 2008, 1724 - 1730.